A recurring problem with dispersing solids, such as polymers, into a liquid is that the solids form clumps which are less readily dispersed than small particles. The invention relates to the dissolution of solids in a liquid solvent and dispersion of solids into a liquid in colloid form. It will be described in terms of a particular aspect, the dissolution of polymers into water. Many techniques have been developed for mixing and dispersing soluble polymers in water. Conventionally the polymers are dispersed directly into the water and subjected to mechanically induced agitation in a batch process. See, for example, U.S. Pat. No. 3,807,701. It is also known that good mixing can be achieved in turbulent streams of water. U.S. Pat. No. 3,468,322 is an example of a turbulent mixing system.
Problems are often encountered in the use of conventional mixing techniques to dissolve polymers, especially water-soluble polymers in water. In order to dissolve quickly and easily, uniform initial dispersion of the polymer is essential. To avoid agglomeration of the polymer in the water and resulting slow dissolution rate for the agglomerates, severe mechanical agitation may be used to overcome poor initial dispersion. High shear agitation has the disadvantage that when applied to polymer systems, the molecules may be broken down and the polymer lose some of its desired properties.
In addition to damage caused by high shear, another problem with batch systems is that they involve high power requirements. In order to ensure complete dissolution in a reasonable period of time, excessive agitation is used. This has the problems discussed earlier, i.e. that the solids being dissolved may be damaged. It also leads to excessive use of energy.
Another approach to the dissolution of polymers in water is to predisperse the polymer in a water-miscible non-solvent. The non-solvent liquid helps prevent agglomeration of the polymer particles until complete dissolution has occurred. This method is often not desirable because the additional non-solvent liquid must be removed, an unnecessary additional step. Even when the non-solvent is not removed, the non-solvent liquid adds to the cost of the operation and requires an additional preparatory step.
A multi-pass continuous method is shown in U.S. Pat. No. 4,664,528. Single-pass, continuous systems for dissolving water soluble polymers also exist. However, their successful operation critically depends on proper initial dispersal of the polymer as with batch systems. These methods use elaborate designs to provide uniform initial dispersal of the polymer but are not well suited to ensure dissolution if the initial dispersal is not uniform for any reason. Batch dissolution systems are more tolerant of poor initial dispersion because agitation overcomes the agglomeration resulting from poor dispersion.
An additional problem with many of the prior art methods of dissolving polymers is that a holding period of from two to ten hours is required to allow the properties of the solution to stabilize. During the holding period, the viscosity of the solution reaches an equilibrium, at which point the solution is ready for use. This step is sometimes referred to as hydrolysis. This stabilization period requires holding tanks in addition to whatever mixing equipment is used and adds an additional complicating factor to running a continuous operation.
Therefore, a need exists for a polymer dissolution system that does not subject the polymer molecules to high shear rates. Such a device must be able to completely dissolve even large clumps of polymer at useful rates while preserving the effectiveness of the polymer. Such a device should also operate with low energy input. It should also ensure dissolution without being critically dependent upon uniform initial dispersal of the polymer in the water. Finally, it should allow for a constant flow of solution without a holding time to allow the solution properties to stabilize.